In electrostatography, an image charge pattern, also referred to as an electrostatic latent image, is formed on an element and is then developed by treatment with an electrostatographic developer containing particles which are attracted to the charge patterns. These particles are called toner particles or, collectively, toner. The resulting toner image is then generally transferred to a receiver such as a sheet of paper and is fused, or fixed, to the receiver by the application of heat and pressure.
Toner consists primarily of a binder polymer. In order to fuse the toner image onto the support it is necessary to elevate the temperature of the toner above the Tg, the glass transition temperature, of the toner binder, at which point the toner becomes tacky and flows, to an extent, into the fibers or pores of the support member. As the toner cools, solidification of the toner causes the toner image to be firmly bonded to the support.
Several approaches to thermal fusing of toner images have been described in the art. These methods include providing the application of heat and pressure at substantially the same time by passage through a nip defined by a pair of opposed members, such as: a pair of rolls maintained in pressure contact; a flat or curved plate mender in pressure contact with a roll; or a belt member in pressure contact with a roll. Heat may be applied by heating one or both of the members. Adequate fusing of the toner image calls for a proper combination of heat, pressure, and contact time. The balancing of these parameters for particular apparatus and process conditions is well known in the art.
Members of the fuser are commonly referred to as a "fusing member", a "fusing roll" or the like, and a "pressure member" or "pressure roll" or the like. The fusing member contacts the toner image, while the pressure member contacts the opposite surface of the receiver. Multiple members of each type are, of course, possible, as are fusers in which opposed fusing members simultaneously fuse toner images on both sides of a single receiver.
During fusing, it is important that toner particles not transfer from the toner image on the receiver to a fusing member. Toner particles offset onto the fusing member may deposit on a subsequent receiver resulting in a false or "offset" image or may contaminate the pressure member, or may eject onto other portions of the apparatus. The net result is a degradation of the copying cycle. What is commonly referred to as "hot offset" occurs when the temperature of toner is raised to a point where toner particles liquefy, and the toner image splits with a portion remaining on the fusing member. The temperature of hot offset is a measure of the release property of a fusing member.
The hot offset temperature of a fusing member may be increased by covering the surface of the fusing member with a low surface energy material such as a silicone elastomer or tetrafluoroethylene resin. Such materials have a further advantage in that contaminants, such as paper fibers and other debris, which would cause a reduction in hot offset temperature, do not readily adhere. Depending upon the resiliency and other characteristics of the low surface energy layer, a fuser member may also include a base cushion of resilient material between the support and the low surface energy layer. The different silicone elastomer layers generally adhere to each other reasonably well.
Release agents, such as silicone oil, are commonly used on fusing members to further insure complete release of the toner image. Silicone release agents can cause swelling of silicone elastomers. This is undesirable and can result in failure of the silicone elastomer. A number of remedial measures are known, for example, silicone fluoride elastomer is resistant to silicone oil.
A fuser roll of cured silicone molding compound is described in Research Disclosure, January 1991, Item 32119. It is indicated that the fuser roll can be fabricated by the process of liquid injection molding. The silicone molding compound is Silastic J, a two-part liquid silicone elastomer which is cross-linked via an addition reaction between vinyl and hydride functionalities, accelerated by a platinum catalyst. The user roll is described as being resistant to premature failure due to fluid (silicone oil) absorption.
It is common practice to internally heat at least one member of a pair of fusing members. There are advantages, however, such as decreased energy consumption and quicker warm-up, that can be provided by the less common practice of externally heating at least one fusing member. An externally heated fusing member is taught by Research Disclosure, January 1991, Item 321118, published by Industrial Opportunities Ltd., Homewell Hayant, Hampshire, P09 1EF, UK, (this publication is hereafter referred to as Research Disclosure). With an internally heated fusing member, it is desirable to maintain the entire fusing member at a substantially uniform temperature, thus coatings are selected which dissipate heat well. With an externally heated fusing member, it is desirable to heat only the outer portions of the fusing member, since the core acts as a heat sink. It is thus desirable to provide a heat insulating layer over the core and a heat dissipating layer over the heat insulating layer. There is a thermal gradient in such a fusing member which has increased thermal stresses relative to an internally heated fusing member..
It is desirable to provide an improved fusing member and method for preparing an improved fusing member in which a heat insulating layer is bonded to a metallic support.
There has long been an interest in adhering materials to aluminum and a number of procedures have been developed for that purpose. In Surface Analysis and Pretreatment of Plastics and Metals, ed. D. M. Brewis, Macmillan Publ. Co., Inc., New York, chapter 8: "Surface Treatments for Aluminum", (1982); procedures are divided into classes: mechanical treatment, alkaline cleaning, chemical etching, and acid anodizing. U.S. Pat. Nos. 3,383,249; 3,400,021; 3,985,584; 4,227,946; 5,053,081; and 5,158,622 describe various chemical treatment procedures.
Chemical etching is commonly also referred to as chemical conversion coating and is defined in the Metals Handbook, 9th edition, ASM ed., Volume 5, "Surface Cleaning, Finishing, and Coating", ASM, Metals Park, Ohio, p. 597:
"Chemical conversion coatings are adherent surface layers of low-solubility oxide, phosphate, or chromate compounds produced by the reaction of suitable reagents with the metallic surface."
Chromate conversion coating defines well known procedures commonly utilized to treat metals such as aluminum. Chromate conversion must be distinguished from chromic acid anodizing. The processes themselves utilize a chemical oxidation-reduction reaction, in the case of the conversion process and an electrochemical reaction, in the case of the anodization. The coatings produced are also very different:
"Chromic acid anodizing produces a thick, dense oxide which consists of solid columns approximately 400 mm in diameter with a smooth surface. This thick oxide improves the corrosion resistance but the lack of porosity reduces the possibility of mechanical interlocking and hence it might be expected that the initial strengths of adhesive bonds would be lower. This explanation was supported by a recent study which compared chromic acid etching and anodizing. The etched adherents gave higher initial bond strengths than those which had been anodized but the latter gave more durable bonds in a high humidity environment." Surface Analysis and Pretreatment of Plastics and Metals, cited above, p. 182-183. Metals Handbook, cited above, at p. 597 similarly states:
"Conversion coatings are used interchangeably with anodic coatings in organic finishing schedules. One use of conversion coating is as a spot treatment for the repair of damaged areas in anodic coatings. Conversion coatings should not be used on surfaces to which adhesives will be applied because of the low strength of the coating. Anodic coatings are stronger than conversion coatings for adhesive bonding applications."
U.S. Pat. No. 4,822,631 to Beaudet, teaches an imaging member (a member on which the latent electrostatic image is created and toned), which has an aluminum cylinder, smoothed and then anodized to produce an oxide surface. After anodizing, which is not described in any detail, the aluminum cylinder is rinsed in deionized water, baked, and then covered with a layer of silicon rubber or the like.
U.S. Pat. No. 4,196,256 teaches a fuser roll having a flame sprayed metal surface of steel, stainless steel, nickel, nickel/chromium, molybdenum or the like optionally covered with a primer containing chromic acid, phosphoric acid, tetrafluoroethylene and water and then covered with a fused powder coated copolymer of perfluoroalkyl perfluorovinyl ether and tetrafluoroethylene. A preferred metal for the flame sprayed layer is number 304 stainless steel wire.
Therefore, it would be desirable to utilize a chemical treatment to improve adhesion of a silicone layer to an aluminum support in order to provide an improved fusing member and method for preparing an improved fusing member.